1. Field of the Invention
The invention is directed to an improved fuel injection apparatus for an internal combustion engine of the type having a high pressure fuel pump and a fuel injection valve for each cylinder of the engine.
2. Description of the Prior Art
A fuel injection apparatus of the type with which this invention is concerned is known from European Patent 0 987 431 A2. This fuel injection apparatus has a high-pressure fuel pump and a fuel injection valve for each cylinder of the internal combustion engine. The high-pressure fuel pump has a pump piston that is driven into a stroke motion by the engine and that delimits a pump working chamber. The fuel injection valve has a pressure chamber connected to the pump working chamber and an injection valve element that controls at least one injection opening and, actuated by the pressure prevailing in the pressure chamber, can be moved in the opening direction counter to a closing force in order to open the at least one injection opening. A control valve is provided that controls a connection of the pump working chamber to a relief to chamber and a pressure source. When the control valve is open, the pump working chamber is filled with fuel from the pressure source during the intake stroke of the pump piston. It is desirable for the high-pressure pump to produce a high-pressure even at low speeds of the engine, permitting a high output and powerful torque of the engine to be achieved. The pressure produced by the high-pressure pump, however, increases with the speed of the engine; the maximum pressure produced must be limited in order to assure a sufficient service life of the high-pressure pump. A design compromise must therefore be struck between a specified drive unit of the high-pressure pump and a specified diameter of the pump piston in order on the one hand to achieve a sufficiently high pressure at a low engine speed and on the other hand not to exceed the maximum pressure that has been specified for reasons related to the service life. The injection valve element of the fuel injection valve controls an injection cross-section that is always the same size. This does not permit an optimal fuel injection under all operating conditions of the internal combustion engine.